How do I drive large loads with the "T"

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When you want to drive generic devices such as lamps, motors, or anything else using the TRS-80, it is necessary to build a little interface that allows the programmer to turn on and of a number of relays connected to each device.

There are two possible solutions: to build a completelly new interface that connects directly to the bus of the T, or to use the parallel port instead; in both instances, it is necessary to include a stage that actually drives the relays.

In my experience I have noted that microprocessors in general are extremely sensitive to Electromagnetic interference, or better, not so much the chips themselves, but the whole system of traces and components around them that act like antennas and generate noise that can cause malfunctioning of the CPU. One of the most certain ways to allow evil into your system is to share the ground between your computer and your power interface. It does not really matter how clean you think your ground is, I recommend you stay away from this as much as you can. How do you do that?

 

Relays are usually a pretty good way to separate the actual "load" from the computer lines, but if you need to drive a washing machine (no kidding), you need a pretty powerful relay that might just as well be "heavy" to deal with for your poor 8085 and friends.

What I recommend is to always use OptoIsolators (4N35, TLP521...), separate power supplies, RF chokes, Triaks when possible instead of relays.

Here, the parallel port (or output interface) drives the LED inside the OptoIsolator in sink mode: when the line is low, the LED turns on. The +5v comes directly from the computer's power supply, from batteries, or from a separate PSU. Because the current in this branch is pretty small (about 10mA), there is no problem with noise and interferences. The stage to the right uses a separate power line that should come from an independent power supply unit. Its voltage depends on the relay's needs. Depending on the size of the relay's coil, the reverse currents generated when the relay turns off (shunt by D1) can cause spikes on the power lines. If needed, insert RF chockes and filters on + and - to reduce them. At last, the LOAD (lamp or so) is powered by whatever you need. Again, for large loads, use filters to suppress dangerous spikes.

Again, the parallel port (or output interface) drives the LED inside the OptoIsolator in sink mode: when the line is low, the LED turns on. The +5v comes directly from the computer's power supply, from batteries, or from a separate PSU. Because the current in this branch is pretty small (about 10mA), there is no problem with noise and interferences. The stage to the right uses a Triak to directly drive the load by the main power that must be AC (otherwise once lit, the load will be on until you disconnect it). Again, for large loads, use filters on the main lines to suppress dangerous spikes.

Driving The Lines

Now is the time to start sending data to our interface. If we use our own interface card, then we need to know at what I/O address it is mapped, then perform an [ OUT (address),data ]from Basic. If we use the parallel port, it is just as easy, use [ LPRINT CHR$(data); ] instead. Don't forget the ";" after LPRINT or the system will send 10 and 13 also as "End of line" and "Carriage Return".

Data is a number between 0 and 255; in binary each one of the 8 bits controls one of the lines. According to your diagram, either a 0 or a 1 will activate each load. For our interfaces above, we need to send out a "0" to turn one relay (or triak) on.

Start from something like this:

FOR I=0 TO 7

DATA=2^I

LPRINT CHR$(255-DATA);

FOR DELAY=0 TO 200:NEXT DELAY

NEXT I

 

Remember you can use AND to reset line "N" and OR to set it: (N BETWEEN 0-7)

 

DATA=DATA OR (2^N)

LPRINT CHR$(255-DATA);

 

DATA=DATA AND (2^N)

LPRINT CHR$(255-DATA);

 

I hope this gives you ideas to start your own interfacing project. Good Luck.